An X-ray computed tomography apparatus uses air, various types of phantoms, and the like for the calibration of a detector. In general, an RI (Radioactive Isotope) is often used for the calibration of the gain of the detector. In order to calibrate a photon counting CT detector by using an RI, however, it is necessary to use an RI having a radioactivity of 100 MBq or more. This is practically difficult. A detector varies in properties depending on external environments such as temperatures. In addition, in photon counting CT, since data is processed for each energy bin having a predetermined energy width, it is necessary to accurately calibrate the gain of a device.